Japan braced for a full-scale nuclear catastrophe today after a new fire at one reactor and damage to another sharply raised the chances of a meltdown and government officials admitted radiation was spewing into the environment.

Radiation levels within the apparently doomed Fukushima Daiichi nuclear plant surged to such dangerous levels at around 11:30 a.m. local time today that the last 50 workers trying to contain the disaster had to be temporarily evacuated.

Earlier, officials ordered 140,000 people around the plant to seal themselves inside their homes.

Officials said the new blaze started because yesterday’s had not been fully extinguished.

Today’s fire was apparently extinguished after burning for about three hours, but authorities weren’t able to confirm that.

White smoke was later spotted rising out of the No. 3 reactor. It may have been caused by an increase in the temperature of the fuel pool, the plant’s operator said.

The IAEA raised the Japanese crisis to Level 6 on the international scale of nuclear disasters, officially making it worse than the Pennsylvania Three Mile Island accident of 1979 and second only to the Ukrainian Chernobyl catastrophe of 1986, which hit Level 7, the highest on the scale.

The operator of the tsunami- and earthquake-ravaged plant said spraying water and boric acid from helicopters to prevent further leakage is being considered.

But that not be enough, one expert warned.

“If I had the ear of the Japanese prime minister, I would suggest . . . [sandbagging] the reactors, entombing it in a sarcophagus of concrete,” CUNY physicist Dr. Michiko Kaku told ABC News.

“That was the last resort that the Soviets used in 1986.”

For a while, it appeared there would be no one left to attempt any “last resort,” when all the crew members were evacuated.

“The workers cannot carry out even minimal work at the plant now,” said Edano.

The employees were pulled out when readings inside the plant spiked to a level high enough to cause radiation sickness.

They were allowed back in several hours later.

A US nuclear expert said the evacuation — which meant there was nothing actively preventing a full-scale meltdown — was “a surrender.”

“It’s basically a sign that there’s nothing left to do but throw in the towel,” said David Lochbaum, head of US nuclear-safety group, the Union of Concerned Scientists.

The plant had earlier ordered the other 800 or so workers to leave. Two workers were reported missing and 15 were injured after the explosion at the No. 4 reactor.

Among the day’s other bad news:

* Financial markets took another pounding. The Dow Jones industrial average dropped nearly 300 points in early trading but ended the day down 137 points. Japan’s Nikkei index fell another 10.6 percent yesterday, completing a two-day loss of $620 billion in stock value.

But it opened 6 percent higher today and the Bank of Japan announced it was injecting another 3.5 trillion yen, or $43 billion, into the economy.

Late in the session it was up 5.75 percent.

* More US Navy crew members were exposed to radiation yesterday and had to be decontaminated aboard their ship, the USS Ronald Reagan, after taking part in helicopter search-and-rescue operations.

* A former engineer at General Electric said he resigned from the company 35 years ago over design flaws in the type of reactor at the Daiichi plant. Dale Bridenbaugh said the “Mark 1″ design wasn’t capable of withstanding a large-scale accident, although he believes the errors were corrected at Daiichi.

* Europe’s 500 bone-marrow transplant centers were alerted to be ready to treat large numbers of Japanese radiation victims.

There are fears that the Japanese are losing control of many of the reactors in the complex.

Reactor No. 4, site of the latest meltdown threat, had drawn little attention before yesterday because it was shut down for maintenance before last week’s earthquake and tsunami.

It was used to keep spent nuclear-fuel rods cool. But officials said the 30 feet of water normally covering the fuel may be boiling. If the water evaporates, the fuel would be exposed, setting off catastrophic nuclear reactions.